1. Field of the Invention
The present invention relates to a method for testing the surface status of a sample (a semiconductor device, etc.) and apparatus therefor and particularly to an inspection method and apparatus therefor for imaging and inspecting fine pattern defects on the surface of a semiconductor device in high sensitivity and high resolution at high speed using an electron beam.
2. Description of the Prior Art
As an inspecting method for detecting defects of a circuit pattern formed on a wafer by comparison test in the manufacturing process of a semiconductor device, there is a method for obtaining images of two or more same kind of LSI patterns on one wafer using light, comparing these plurality of images, and testing existence of pattern defects and it is already put into practical use. The outline of this inspecting method is described in xe2x80x9cMonthly Semiconductor Worldxe2x80x9d, October issue, 1995, pp. 114 to 117. When pattern defects in the manufacturing process of a semiconductor device are tested by such an optical inspecting method, residuals of a silicon oxide film through which light transmits and a photosensitive resist material cannot be detected. Residual etching below the resolution of the optical system and a nonopening defect of a fine conducting hole can be neither detected.
To solve such a problem in the optical inspecting method, a pattern comparison inspecting method using an electron beam is described in Japanese Patent Application Laid-Open 59-192943, J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3005-3009 (1991), J. Vac. Sci. Tech. B, Vol. 10, No. 6, pp. 2804-2808 (1992), SPIE, Vol. 2439, pp. 174 0 183, and Japanese Patent Application Laid-Open 05-158703. In this case, to obtain a practical inspecting speed, it is necessary to obtain pattern images at a very high speed. To reserve the S/N ratio of images obtained at high speed, a beam current more than 100 times (more than 10 nA) of that of a normal scanning electron microscope is used.
In the aforementioned prior testing art using an electron beam, to form images maintaining the S/N ratio which can be tested, an electron beam having a large current is used. However, since the electron beam is limited to a spot shape and this spot beam is two-dimensionally scanned on the surface of a sample, there is a limit to the high speed (shortening of the inspecting time). There is also a limit to the large current of an electron beam to be used due to the brightness of the electron source used and space charge effect. For example, to obtain a resolution of about 0.1 xcexcm, the electron beam current to be used is theoretically limited to about several hundreds nA and only about 100 nA can be actually used. The S/N ratio of an image is decided by the number of electrons to be used so as to form the image, that is, the product of beam current and time required to obtain the image. In consideration of necessity of reservation of the S/N ratio on the image processing ready level, to obtain a resolution of 0.1 xcexcm at a beam current of 100 nA, about 100 seconds or more are required to test an area of 1 cm2 of the surface of a sample. On the other hand, in the aforementioned conventional optical inspection apparatus, the test required time for an inspection area of 1 cm2 is very short such as about 5 seconds.
Therefore, an object of the present invention is to shorten the test required time of the pattern comparison inspecting method using an electron beam such as equal to or less than that of the conventional optical inspecting method.
To accomplish this object, the present invention is a pattern defect inspecting method and apparatus therefor for spreading and irradiating an electron beam from the electron source to a fixed area region on the surface of a sample at the same time, imaging backscattering electrons obtained from the area region or secondary electrons and forming an enlarged image of the area region, moving the sample so as to irradiate the electron beam at the desired location of the surface of the sample, converting the enlarged image of the area region formed by the aforementioned image forming means to an image signal, and comparing the image signal of one area region on the surface of the sample obtained by the aforementioned image signal obtaining means with an image signal of another area region and detecting a pattern defect in the one area region.
Furthermore, the pattern defect inspecting method and apparatus therefor of the present invention is characterized in that the method and apparatus include at least first electron beam irradiation for spreading and irradiating an electron beam from the electron source to a first area region on the surface of a sample at the same time, first electron image forming for imaging backscattering electrons emitted from the first area region or secondary electrons and forming a first electron image of the first area region, first image signal obtaining for obtaining an image signal of the first electron image of the first area region, irradiation position movement for moving the electron beam irradiation position from the first area region on the surface of the sample to a second area region, second electron beam irradiation for spreading and irradiating the electron beam from the electron source to the second area region on the surface of the sample at the same time, second electron image forming for imaging backscattering electrons emitted from the second area region or secondary electrons and forming a second electron image of the second area region, second image signal obtaining for obtaining an image signal of the second electron image of the second area region, and defect detection for comparing the image signal of the first electron image of the first area region obtained at the aforementioned first image signal obtaining stage with the image signal of the second electron image of the second area region obtained at the aforementioned second image signal obtaining stage and detecting a pattern defect in the first area region or the second area region.
More concretely, the above object of the present invention, that is, speeding-up of the pattern comparison inspecting method using an electron beam is realized by sequentially irradiating an electron beam to a plurality of irradiation regions (area regions) of the surface of a sample as an area beam having a two-dimensional spread instead of a spot beam, sequentially forming enlarged images of the plurality of irradiation regions by imaging backscattering electrons from the plurality of irradiation regions (area regions) or secondary electrons, converting the enlarged images of the plurality of irradiation regions to electrical image signals, and detecting a pattern defect in each of the aforementioned irradiation regions by comparing the image signals in the plurality of irradiation regions with each other.
Namely, in the patterned wafer inspection method of the present invention, an electron beam from the electron source is sequentially irradiated to a plurality of irradiation regions (area regions) of the surface of a semiconductor sample as a so-called area beam, and enlarged images in the plurality of irradiation regions are sequentially formed by electron-optically imaging backscattering electrons from the plurality of irradiation regions or secondary electrons, and the enlarged images in the plurality of irradiation regions are sequentially converted to electrical image signals and stored, and a pattern defect in each of the aforementioned irradiation regions is detected by comparing the stored image signals in the plurality of irradiation regions with each other. According to this method, the conventional two-dimensional scanning by a spot beam in each irradiation region (area region) is not necessary, so that the inspection time can be greatly shortened and the defect test can be speeded up.
The patterned wafer inspection apparatus of the present invention can comprise an electron optical system for irradiating an electron beam from the electron source to the surface of a semiconductor sample as an area beam and forming an enlarged image in the irradiated region by imaging backscattering electrons from the irradiation region (area region) or secondary electrons, a sample moving stage for loading the semiconductor sample and moving the semiconductor sample so that the electron beam is irradiated at the desired position on the surface of the semiconductor sample, an image signal detection means for converting and detecting the enlarged image to an electrical image signal, and an image signal processing means for detecting a pattern defect in each irradiation region by comparing the image signal in a plurality of irradiation regions on the surface of the semiconductor sample detected by the image signal detection means.
It is valid to decelerate the electron beam irradiated onto the sample surface by applying a negative potential to the sample and let the decelerated electron beam enter the sample surface or reflect from the neighborhood of the pole without entering the sample surface.
When the sample moving stage is set so as to continuously move the sample at almost uniform velocity, more speeding-up of the defect inspection can be realized. In this case, needless to say, by providing a stage position monitoring means for monitoring the position of the sample moving stage, it is necessary to control so that the electron beam irradiation region on the sample surface is kept at the same position on the sample surface for a predetermined time.
Furthermore, the image signal detection means converts the enlarged electron image of the irradiation region imaged and formed by the electron optical system to an optical image by projecting it onto the fluorescence plate and images the optical image on the optical image detection device via the optical lens or optical fiber. Or, the enlarged electron image formed by the electron optical system may be directly formed on the image detection device having electron sensitivity. As an image detection device, a charge coupled device (CCD sensor) or a device for integrating and outputting an optical signal inputted with the time delayed can be used. To read a detection signal from the image detection device, a system for reading by many channels in parallel is used.
On the other hand, a method for setting so that the size of enlarged images on the surface of a semiconductor sample which can be obtained at the same time by irradiating an electron beam at the same time becomes almost equal to the size of the light receiving surface of the image detection device is simpler. On the other hand, by setting the size of the electron beam irradiation region so that the size of enlarged images on the surface of a semiconductor sample is made smaller than the size of the light receiving surface of the image detection device, scanning the electron beam on the surface of the semiconductor sample, then projecting the enlarged images overall the light receiving surface of the image detection device for a given period of time, and superimposing a signal for correcting the variation factors of irradiation position and irradiation range on the scanning signal of the electron beam, a method realizing higher precision may be available.
To decelerate an electron beam to be irradiated onto a semiconductor sample, make the energy value of the electron beam when it is irradiated onto the sample sufficiently smaller than the energy value before deceleration, and keep the energy dispersion of backscattering electrons generated from the sample surface by irradiation of the decelerated electron beam within a range that it will not affect the resolution of the imaging system, a negative potential is applied to the semiconductor sample. Or, by providing a filter for discriminating backscattering electrons generated by irradiation of the electron beam or secondary electrons in energy and imaging only backscattering electrons or secondary electrons with a specific energy width, the problem of high speed test can be solved and the resolution can be improved at the same time.
The foregoing and other objects, advantages, manner of operation and novel features of the present invention will be understood from the following detailed description when read in connection with the accompanying drawings.